In the metal fabrication industry, especially the automotive industry, it is necessary to achieve high dimensional precision when fabricating metal parts. For example, die cast metal parts are made for engines by forcing molten metal into a water-cooled steel mold. During cooling, the metal will "flash" between the two mold halves due to the high pressure used in the process. This flash must be trimmed off before the part is subjected to finish machining or attached to the final product and shipped to the customers.
Flash on a cast metal part is usually trimmed by the use of a device known as a trim press. A trim press consists of punches and die cores that work to remove the flash. The punches remove flash from openings in the cast metal part, while the die cores are adapted to create high precision surfaces on the casting. The cast part is placed in the trim press and then the press descends vertically with a momentum sufficient to shear and remove the flash with a punch. The traditional punch accomplishes this by self-aligning with the opening, by moving laterally relative to its vertical axis, and the press plate upon which it is mounted. A small amount of lateral movement is required to prevent damage to the cast part while flash is being removed so that certain tolerances can be maintained in desired dimensions of the cast part. After being cycled through the trim press, the part is inspected. The part will be rejected if too much flash remains. Depending on the particular application, most of the flash must be removed to meet predetermined tolerances. Typically, the parts are then fitted into final assemblies or machining jigs or pallets for further processing. Thus, in certain manufacturing environments, a part can be rejected if there is more than, for example, about 0.5 millimeters (mm) of flash encircling the inside of the opening in the cast part, or if the remaining flash is uneven, because the part will not fit on a machining pallet.
In removing the flash from cast parts, it is necessary to avoid damage to the part. In particular, damage can occur when removing flash from openings in the cast part. Such damage can occur as the cutting edge of a trim punch is moved into the opening for shearing and removal of the flash. Generally, the damage occurs from contact of the trim punch cutting edge with the cast part, or the sides of the opening, or both.
This damage may occur in several situations. When the maximum outside dimensions of the trim punch cutting edge exceeds the minimum possible dimensions of the cast part opening, the punch will contact and damage the part. Damage can also occur when the positional location of the cast part opening is slightly offset. In this instance, the trim punch will not be centered upon the opening.
In any of these situations, the trim punch cutting edge can be forced into undesired contact with the cast part. To avoid these problems, it is known to select a trim punch cutting edge having maximum possible dimensions that are always less than the minimum possible dimensions of the opening in a cast part. It is also known to select a trim punch that can move horizontally relative to the centerline of the opening so as to align the trim punch with the cast part opening during the trimming operation. See, for example, U.S. Pat. No. 5,715,721. However, each of these methods requires an additional machining operation because all of the flash cannot be removed in a single fabrication step. Moreover, due to unavoidable manufacturing errors, such as the aforementioned opening oversize and undersize dimensions, positional tolerance stack-up and cast part opening misalignment, the flash will be removed unevenly relative to the centerline and the side walls of the cast part opening.
Previously, prior art devices have been directed to the low-precision initial removal of a large portion of the flash accompanied by subsequent additional high-precision manufacturing steps directed to the careful removal of the remaining flash to meet selected, acceptable tolerances. Accordingly, what is needed, but heretofore unavailable, is a trim punch that can evenly remove as much flash as possible without damaging the cast part and/or the opening in one step.
The vehicle industry is very competitive; and if the quality of cast parts is compromised, or if attaining the desired quality level is inefficient or unnecessarily expensive, the manufacturer suffers economically. The current system for removing flash from a die cast metal part is inadequate as an unacceptable high level of rejected parts is experienced. The self-centering trim punch of the present invention provides a solution to the problem of an excessive or uneven flash distribution on a metal casting. Dimensional tolerances of approximately 0.5 mm or less of flash remaining can be achieved using the inventive punch.